Much of the research and development effort in the metal coating art is directed to the search for coating materials and methods of applying such materials which eliminate or approach elimination of volatile, organic solvents released in heat curing, which produce coatings at least comparable to conventional paints and methods of painting in appearance and durability, and which can be produced at a commercially feasible cost.
One proposal before the art is to replace liquid coating material with coating materials in the form of so-called water-based coatings, i.e., aqueous resin solutions and aqueous resin emulsions. Conventionally, these contain a concentration of volatile organic solvents that is far below that in conventional liquid enamels and lacquers, i.e., resin solutions and resin dispersions or both in an organic solvent, but significantly higher than is found in powder coatings. Other problems encountered with water-based coatings include (1) problems of humidity control (2) problems of film fracture during the bake known as "popping", occur in areas receiving an unnecessarily thick coating, often the result of substrate contours, and (3) problems in obtaining finish coatings having a high gloss without special care and cost in formulation.
Another approach to providing quality coatings in a low emission system has been the use of the so called "powder paints." These conventionally contain very low concentrations of volatile solvents, i.e., substantially less than any other paint system and of the order of 2& or slightly higher, and, in this regard, have much to recommend them. Inherent in their use, however, are certain problems of production and application which have retarded the extent of their adoption. One of these involves their use in conjunction with particulate metal pigments, ordinarily aluminum flakes. Automobiles coated with a so-called "metallic" finish, i.e., a topcoat of enamel or lacquer in which there is dispersed aluminum flakes in addition to conventional pigments, have found wide acceptance in the marketplace. For the most part, the problems incidental to employing aluminum flakes in conventional liquid paints, i.e., problems of paint manufacture and paint application, have been solved through years of experimentation and use. The problems of using aluminum flakes in dry powder are far more complex, particularly where some type of pulverizing step is a part of the paint manufacturing process and when application of the paint to a substrate is by electrostatic spray techniques. Further, while increased use of powder coatings in greater volume and improved methods of manufacture will reduce the present cost of quality powder coatings, their production in all of the colors demanded in the marketplace may be prohibitive.
One approach to resolution of these problems involves the application of a highly pigmented, metal-flake containing water-based enamel base coat, which is subsequently baked and then topcoated with a clear powder coating. While this process has many advantages and produces an excellent surface finish, it necessitates handling two different paint systems each of which require quite different application and handling processes and equipment.